The invention relates to a process and system to reduce emissions of nitrogen oxides and mercury and to reduce the level of carbon in combustion fly ash. More specifically, the present invention provides a process and system to increase use of fly ash and to decrease nitrogen oxides and mercury from flue gases from combustion systems such as boilers, furnaces and incinerators.
Production of air pollution by combustion systems is a major problem of modern industrial society. The pollution can include particulates such as fine fly ash particles from solid fuel combustion (for example, pulverized coal firing), and gas-phase species, such as oxides of sulfur (SOx, principally SO2 and SO3), carbon monoxide, volatile hydrocarbons, nitrogen oxides (mainly NO and NO2 collectively referred to “NOx”) and volatile metals such as mercury (Hg).
The nitrogen oxides are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. NOx is emitted by a variety of sources, including mobile sources (such as automobiles, trucks and other mobile systems powered by internal combustion engines), stationary internal combustion engines and other combustion sources such as power plant boilers, industrial process furnaces and waste incinerators. Available NOx control technologies include Selective Catalytic Reduction (SCR) and Combustion Modification. SCR systems can be designed for most boilers and may be the only approach for high NOx units such as cyclones. Combustion Modification achieves deep NOx control by integrating several components. Typically, Low NOx Burn (LNB) is the lowest cost Combustion Modification technique. It is usually applied as a step towards low cost deep NOx control. Other Combustion Modification techniques include Overfire Air (OFA), Reburning and Advanced Reburning.
Mercury is identified as a hazardous air pollutant and is the most toxic volatile metal in the atmosphere. Elemental mercury vapor can be widely dispersed from emission sources. Other forms of mercury pollutants include organic and inorganic compounds that accumulate in plants and animals. Mercury is a constituent part of coal mineral matter. Its emission from coal-fired power plants is suspected to be a major source of environmental mercury.
Thus, there is a need to continue to use low NOx technologies but to effectively control mercury emission.